The present invention is directed to bicycle shift control devices and, more particularly, to bicycle shift control devices which employ a return spring in the transmission (such as a derailleur) and an assist spring in the shift control lever.
A conventional shifting control device is provided with an assist spring for movably energizing the shifting control piece in the direction opposite to the direction in which the piece is energized by a return spring in the transmission (such as a derailleur). The assist spring assists the shifting operation by partially compensating for the cable tension created by the return spring.
In such a shifting device, the operating force needed for down-shifting is the force obtained by adding the chain guide reaction force of the shifting control device and the frictional force of the control cable to the force obtained by subtracting the cable tension generated by the assist spring from the cable tension generated by the return spring. The operating force needed for up-shifting is obtained by adding the frictional force of the control cable to the cable tension generated by the assist spring. Down-shifting is accompanied by an increase in the spring force of the return spring and a reduction in the spring force of the assist spring, whereas up-shifting is accompanied by a reduction in the spring force of the return spring and an increase in the spring force of the assist spring.
FIGS. 13 and 14 show the relation between the tension CT1 exerted by the assist spring on the control cable and the tension CT2 exerted by the return spring on the control cable in some devices when, for example, the return spring of such a conventional shifting device energizes the derailleur in the direction of the top speed. As shown in FIG. 13, the difference between the operating force required for shifting on the low side and the operating force required for shifting on the top side can be quite substantial. A known shifting device force required for shifting on the top side can be quite substantial. A known shifting device having the tension relation shown in FIG. 14 has a smaller difference than does the shifting device having the tension relation shown in FIG. 13. While this is some improvement, a difference still exists between the operating force required for shifting on the low side and the operating force required for shifting on the top side. Furthermore, it should be noted that the tension difference varies depending upon the position of the shifting device and the transmission. More specifically, the difference decreases until the tension curves intersect, and then it increases again. This difference can be distracting to cyclists engaged in high performance tiding, since there is a risk that the cyclist will underestimate or overestimate the shifting force required to go from one gear to another. This can result in missed shifts or incorrect shifts.
It should also be noted that in each case the cable tension CT2 generated by the return spring and the cable tension CT1 generated by the assist spring differ for some or all of the positions lying between the low position and the top position. Thus, if the initial positional relationship between the shift control device and the transmission changes due to stretching of the transmission cable or for some other reason, then tension characteristics will change as well. This could cause the entire system to no longer conform to design specifications. Also, it could further confuse cyclists who engage in high performance riding, since then the tension characteristics of the shifting device depend on how the transmission was set or adjusted by the bicycle mechanic, the amount of wear in the transmission, etc.
Sometimes the derailleur is configured in such a way that the tension exerted by the return spring on the control cable is constant or virtually constant irrespective of the movement of the guide pulley. As shown in FIG. 15, the combined operation of the derailleur and shift control device still brings about a comparatively large increase in the difference between the cable tension CT1 exerted by the assist spring and the cable tension CT2 exerted by the return spring on the low-speed side. While such devices reduce the difference between the operating force exerted during shifting to a lower speed on the low-speed side and the operating force exerted during shifting to a lower speed on the high-speed side in comparison with FIG. 13, again a difference still exists.